CP BIO: Ch. 7 The Cell Membrane and transport across the membrane

Boundary between the cell and its environment The Plasma membrane Boundary between the cell and its environment FLUID MOSAIC MODEL - Describes structure of cell membranes “mosaic” – sea of lipids with scattered proteins “fluid” – molecules float and move around within the layer

Phospholipid Phospholipid = lipid molecule with phosphate on one end glycerol + two fatty acids + phosphate group phosphate group = polar end (hydrophilic “head”) Fatty acids = nonpolar end (hydrophobic “tails”)

Phospholipids form a double layer Nonpolar tails are on the inside (away from water) Polar heads are on the outside ( touch water)

Features of the Cell Membrane Semipermeable = some substances can pass through - some cannot - depends on: molecule size, charge, polar or nonpolar   Cholesterol – molecules scattered among the phospholipids - in animal cells - keep membrane flexible in changing temperatures Carbohydrates – glucose chains on outside of cell - Identification “tags” - Receptor sites for messenger molecules

Fluid Mosaic Model

Membrane Proteins have many functions Transport Enzyme Receptor Allows a specific Catalyzes a Site for molecule to reaction inside messenger pass through the cell molecule to the membrane attach

Membrane proteins (2) Identification Junctions Structure “SELF” – cell belongs Cells join to Keeps internal in this organism, form tissues, parts organized immunity communicate

How do membranes keep homeostasis? Cell membranes are selectively permeable The lipid layer blocks most substances Some molecules can cross the membrane By passive or active transport Some are too big to cross at all

PASSIVE TRANSPORT USES NO CELL ENERGY Molecules move randomly – spread out until evenly distributed From an area of higher concentration to an area of lower concentration

DIFFUSION Diffusion: movement of particles from an area of high concentration to an area of lower concentration Two areas with different concentrations = concentration gradient Particles move in all directions (random) NET movement is from high concentration to low “Down the concentration gradient”

Diffusion Particles spread out until evenly distributed  equilibrium (homogeneous) AT equilibrium, particles continue moving, but in all directions equally  NO further change in concentration

Cell Membranes allow some particles to cross Particles can diffuse across the lipid bilayer if they are: Small Nonpolar (lipid-soluble) Examples: CO2 , O2, fatty acids can diffuse easily

FACILITATED DIFFUSION Transport proteins “help” particles move across the membrane – DOWN their gradient  PASSIVE transport – no cell energy used Proteins are SPECIFIC each allows only a certain substance to pass Particles can cross by facilitated diffusion if they are: Small POLAR (water soluble) or CHARGED (ions) Examples: H2O, glucose, Ca+2, Cl-

Transport Proteins Pores and Channels – open path through membrane Carrier proteins – take particle on one side of membrane and release on the other

OSMOSIS Diffusion of water across a membrane Important process in cell homeostasis  Water crosses the cell membrane easily - Small enough to pass between lipid molecules Also pass through special proteins, aquaporins

Why osmosis matters Water crosses membrane easily, faster than many solutes Will try to reach equilibrium If NET water moves into or out of cell  changes homeostasis Unequal water on one side of cell membrane = osmotic pressure

TONICITY = osmotic pressure in cells ISOTONIC Equal concentrations of solutes inside and outside cell Equal concentrations of water Water goes in and out of cell at equal rates

Isotonic pressure in cells No NET movement of water into or out of cell Normal water pressure in animal cells Wilted (“flaccid”) water pressure in plant cells

When solute concentration is different on two sides of a membrane lower solute concentration = hypotonic Higher solute concentration = hypertonic Solutes will move down their gradient IF THEY CAN CROSS THE MEMBRANE

Water concentration is OPPOSITE of solutes Low solutes  HIGH WATER concentration High solutes  LOW WATER concentration Water WILL diffuse down its gradient and crosses the cell membrane easily Goes TO whichever side has more solutes

Cells in hypotonic solutions Solutes are lower outside cell, water is higher Water enters cell by osmosis Cytoplasm swells - animal cells: swell, may burst (“lyse”) - plant cells: high osmotic pressure “turgor” - won’t burst - cell wall - “Turgid” – stiff and firm, upright stem

Cells in hypertonic solutions Solutes are higher outside cell, water is lower Water leaves cell by osmosis Cytoplasm shrinks - “plasmolysis” - animal cells: shrivel - plant cells: low turgor pressure - cytoplasm pulls away from cell wall - but cell wall does not shrink

Osmosis in Animal Cells Animal cells like ISOTONIC conditions best

Plant cells like HYPOTONIC conditions best Osmosis in Plant Cells Plant cells like HYPOTONIC conditions best

Contractile Vacuoles Fresh-water protists (like Paramecia or Amoeba) must constantly remove water that comes into the cell by osmosis

ACTIVE TRANSPORT – uses cell energy Two kinds: 1. Molecule transport against the gradient a. from low concentration to high b. pushed across by membrane proteins (pumps) c. uses energy ATP d. small molecules and ions

Why would cells use active transport? 1) To concentrate substances: Examples: kidneys concentrate wastes in urine, - intestine concentrates nutrients in blood 2) To maintain an ion concentration - sodium-potassium pump – allows nerve impulses

2) Bulk Transport – uses energy To bring larger particles into or out of cell Endocytosis = brings material into cell - fold cell membrane around it form a vacuole a. Phagocytosis = “cell eating” - large particles or whole cells - examples: amoeba, white blood cells b. Pinocytosis = “cell drinking” - small folds of membrane take in liquids

Pinocytosis – “cell drinking” Example: small intestine absorbs some water this way

Exocytosis – send OUT of cell Vacuole containing substance fuses with cell membrane  opens to outside of cell - for cell secretions - Examples: hormones from endocrine glands digestive juices from pancreas or intestine